R Value | k Value | Perimeter of a round duct is |

1 | 0.86 | 85% a rectangular duct |

2 | 0.80 | 80% a rectangular duct |

3 | 0.72 | 72% a rectangular duct |

4 | 0.65 | 65% a rectangular duct |

5 | 0.60 | 60% a rectangular duct |

6 | 0.55 | 55% a rectangular duct |

7 | 0.51 | 51% a rectangular duct |

8 | 0.48 | 48% a rectangular duct |